The invention relates to a surface drive that is attached to the stern of a watercraft.
Drives attached to the sterns of watercraft are relatively well-known; they include sterndrives and surface drives. The main difference between the two systems, for example, is that the engine exhaust outlet has to be behind the propeller in sterndrives. When the surface propeller rotates and the propeller is fully submerged, the engine exhaust gas is supposed to be in front of the propeller blades for propeller ventilation (see, for example, U.S. Pat. No. 5,046,975). Moreover, for sterndrives, the propeller thrust is transferred to an underwater unit via axial bearings and to the watercraft in part via trim cylinders and in part via an above-water unit (see, for example, U.S. Pat. No. 3,589,204). For surface drives, the propeller thrust affects the rear of the watercraft, directly in the case of rigid drives and directly in the case of pivoting drives (see, for example, U.S. Pat. No. 4,645,463).
Furthermore, an appropriate distance is needed between the propeller of the surface drive and the stern of the watercraft. An appropriate distance is needed because the efficiency of the propeller when the watercraft travels in reverse declines the closer the propeller is located to the stern of the watercraft because a part of the propeller circumference directs the propeller thrust directly against the stern of the watercrart, thus resulting in a flow loss. A technical solution to this problem can be found in U.S. Pat. No. 4,371,350.
The introduction of a controllable-pitch propeller is problematic in that there is not as much space available in a surface drive as there is in sterndrives as described in U.S. Pat. No. 6,250,979. In addition, a hollow shaft design is required for large seagoing vessels (see, for example, WO 8602901), which incurs a high cost. Moreover, the successive changes in load impacting the adjustment mechanism at each blade immersion and emergence for each revolution of the propeller is considerable due to the changes in spindle force applied to the propeller blades when they rotate about a hub. This necessitates a rigid structure and a safe and secure blade location should the hydraulic system fail.